gitproto: detect an early SSH request in Go, not bash

TestSSHConnPostRPCStreamBodyWaitsForAdvertisement relied on read -t 0, which never reports waiting input in macOS's bash 3.2, so the test passed on main there. The shim now creates a marker file just before the closing flush-pkt, and the request body fails the test if it is read before the marker exists. Fails on main and passes here, on macOS too.